Mangling: handle inverse requirements

Author: kavon

docs/ABI/Mangling.rst

@@ -467,8 +467,92 @@ An ``extension`` mangling is used whenever an entity's declaration context is
 an extension *and* the entity being extended is in a different module. In this
 case the extension's module is mangled first, followed by the entity being
 extended. If the extension and the extended entity are in the same module, the
-plain ``entity`` mangling is preferred. If the extension is constrained, the
-constraints on the extension are mangled in its generic signature.
+plain ``entity`` mangling is preferred, but not always used. An extension is
+considered "constrained" if it:
+
+  - Has any requirements not already satisfied by the extended nominal,
+    excluding conformance requirements for invertible protocols.
+  - Has any generic parameters with an inverse requirement.
+
+Those requirements included in any of the above are included in the extension's
+generic signature. The reason for this additional complexity is that we do not
+mangle conformance req's for invertible protocols, only their absence.
+
+::
+
+  struct S<A: ~Copyable, B: ~Copyable> {}
+
+  // An unconstrained extension.
+  extension S {}
+
+  // Also an unconstrained extension, because there are no inverses to mangle.
+  // This extension is exactly the same as the previous.
+  extension S where A: Copyable, B: Copyable {}
+
+  // A constrained extension, because of the added requirement `B: P` that is
+  // not already present in S.
+  extension S where B: P {}
+
+  // A constrained extension, because of the absence of `A: Copyable`.
+  // Despite also being absent in `S`, absences of invertible protocols
+  // are always mangled.
+  extension S where A: ~Copyable {}
+
+Some entities, like computed properties, rely on the generic signature in their
+`context`, so in order to disambiguate between those properties and
+those in a context where a generic type requires Copyable, which is not mangled,
+we have the following rule:
+
+If the innermost type declaration for an entity has any inverses in its generic
+signature, then extension mangling is used. This strategy is used to ensure
+that moving a declaration between a nominal type and one of its extensions does
+not cause an ABI break if the generic signature of the entity is equivalent in
+both circumstances. For example:
+
+::
+
+  struct R<A: ~Copyable> {
+    func f1() {} // uses extension mangling, just like `f3`
+
+    func f2() where A: Copyable {}
+  }
+
+  extension R where A: ~Copyable {
+    func f3() {}
+
+    func f4() where A: Copyable {} // uses entity mangling, just like `f2`
+  }
+
+  extension R where A: Copyable {
+    // 'f5' is mangled equivalent to 'f2' and 'f4' modulo its identifier.
+    func f5() {}
+  }
+
+For intermediate nested types, i.e., those between the top level and the entity,
+any inverses that remain in at the signature of the entity are mangled into
+that entity's generic signature:
+
+::
+
+  struct X<A: ~Copyable> {
+    struct Y<B: ~Copyable> {
+      // 'g1' uses 'entity' context mangling with and has no mangled signatures.
+      func g1() where A: Copyable, B: Copyable {}
+
+      // 'g2' uses 'entity' context mangling. The requirement `B: ~Copyable` is
+      //mangled into the generic signature for 'g2'.
+      func g2() where A: Copyable {}
+
+      // 'g3' uses extension mangling with generic signature 'A: ~Copyable'.
+      // The mangled generic signature of 'g3' is empty.
+      func g3() where B: Copyable {}
+
+      // 'g4' uses extension mangling with generic signature 'A: ~Copyable'.
+      // The mangled generic signature of 'g4' contains 'B: ~Copyable'.
+      func g4() {}
+    }
+  }
+
 
 When mangling the context of a local entity within a constructor or
 destructor, the non-allocating or non-deallocating variant is used.
@@ -680,12 +764,14 @@ Types
   METATYPE-REPR ::= 'T'                      // Thick metatype representation
   METATYPE-REPR ::= 'o'                      // ObjC metatype representation
 
+  existential-layout ::= protocol-list 'p'                 // existential layout
+  existential-layout ::= protocol-list superclass 'Xc'     // existential layout with superclass
+  existential-layout ::= protocol-list 'Xl'                // existential layout with AnyObject
+
   type ::= associated-type
   type ::= any-generic-type
-  type ::= protocol-list 'p'                 // existential type
-  type ::= protocol-list superclass 'Xc'     // existential type with superclass
-  type ::= protocol-list 'Xl'                // existential type with AnyObject
-  type ::= protocol-list requirement* '_' 'XP'   // constrained existential type
+  type ::= existential-layout                         // existential type
+  type ::= existential-layout requirement '_' requirement* 'XP'   // constrained existential type
   type ::= type-list 't'                     // tuple
   type ::= type generic-signature 'u'        // generic type
   type ::= 'x'                               // generic param, depth=0, idx=0
@@ -925,13 +1011,19 @@ now codified into the ABI; the index 0 is therefore reserved.
 
   generic-param-pack-marker ::= 'Rv' GENERIC_PARAM-INDEX   // generic parameter pack marker
 
+  INVERTIBLE-KIND ::= 'c'  // Copyable
+  INVERTIBLE-KIND ::= 'e'  // Escapable
+
   GENERIC-PARAM-COUNT ::= 'z'                // zero parameters
   GENERIC-PARAM-COUNT ::= INDEX              // N+1 parameters
 
   requirement ::= protocol 'R' GENERIC-PARAM-INDEX                  // protocol requirement
   requirement ::= protocol assoc-type-name 'Rp' GENERIC-PARAM-INDEX // protocol requirement on associated type
   requirement ::= protocol assoc-type-list 'RP' GENERIC-PARAM-INDEX // protocol requirement on associated type at depth
   requirement ::= protocol substitution 'RQ'                        // protocol requirement with substitution
+#if SWIFT_RUNTIME_VERSION >= 6.0
+  requirement ::= 'Ri' INVERTIBLE-KIND GENERIC-PARAM-INDEX          // inverse requirement
+#endif
   requirement ::= type 'Rb' GENERIC-PARAM-INDEX                     // base class requirement
   requirement ::= type assoc-type-name 'Rc' GENERIC-PARAM-INDEX     // base class requirement on associated type
   requirement ::= type assoc-type-list 'RC' GENERIC-PARAM-INDEX     // base class requirement on associated type at depth

include/swift/AST/ASTMangler.h

@@ -72,6 +72,9 @@ class ASTMangler : public Mangler {
   /// If enabled, marker protocols can be encoded in the mangled name.
   bool AllowMarkerProtocols = true;
 
+  /// If enabled, inverses will not be mangled into generic signatures.
+  bool AllowInverses = true;
+
   /// If enabled, declarations annotated with @_originallyDefinedIn are mangled
   /// as if they're part of their original module. Disabled for debug mangling,
   /// because lldb wants to find declarations in the modules they're currently
@@ -450,16 +453,54 @@ class ASTMangler : public Mangler {
                                  GenericSignature sig,
                                  const ValueDecl *forDecl);
 
-  void appendContextOf(const ValueDecl *decl);
+  // A "base entity" is a function, property, subscript, or any other
+  // declaration that can appear in an extension.
+  struct BaseEntitySignature {
+  private:
+    GenericSignature sig;
+    bool innermostTypeDecl;
+    bool extension;
+    std::optional<unsigned> mangledDepth; // for inverses
+  public:
+    bool reachedInnermostTypeDecl() {
+      bool answer = innermostTypeDecl;
+      innermostTypeDecl = false;
+      return answer;
+    }
+    bool reachedExtension() const { return extension; }
+    void setReachedExtension() { assert(!extension); extension = true; }
+    GenericSignature getSignature() const { return sig; }
+    // The depth of the inverses mangled so far.
+    std::optional<unsigned> getDepth() const { return mangledDepth; }
+    void setDepth(unsigned depth) {
+      assert(!mangledDepth || *mangledDepth <= depth);
+      mangledDepth = depth;
+    }
+    BaseEntitySignature(const Decl *decl);
+  };
+
+  void appendContextOf(const ValueDecl *decl, BaseEntitySignature &base);
+  void appendContextualInverses(const GenericTypeDecl *contextDecl,
+                                BaseEntitySignature &base,
+                                const ModuleDecl *module,
+                                StringRef useModuleName);
 
-  void appendContext(const DeclContext *ctx, StringRef useModuleName);
+  void appendContext(const DeclContext *ctx,
+                     BaseEntitySignature &base,
+                     StringRef useModuleName);
 
   void appendModule(const ModuleDecl *module, StringRef useModuleName);
 
+  void appendExtension(const ExtensionDecl *ext,
+                       BaseEntitySignature &base,
+                       StringRef useModuleName);
+
   void appendProtocolName(const ProtocolDecl *protocol,
                           bool allowStandardSubstitution = true);
 
   void appendAnyGenericType(const GenericTypeDecl *decl);
+  void appendAnyGenericType(const GenericTypeDecl *decl,
+                            BaseEntitySignature &base);
 
   enum FunctionManglingKind {
     NoFunctionMangling,
@@ -503,17 +544,42 @@ class ASTMangler : public Mangler {
                                   GenericSignature sig,
                                   const ValueDecl *forDecl = nullptr);
 
+  struct GenericSignatureParts {
+    ArrayRef<CanGenericTypeParamType> params;
+    unsigned initialParamDepth = 0;
+    SmallVector<Requirement, 2> requirements;
+    SmallVector<InverseRequirement, 2> inverses;
+    bool isNull() const; // Is there anything to mangle?
+    bool hasRequirements() const; // Are there any requirements to mangle?
+    void clear();
+  };
+
+  /// Append a generic signature to the mangling.
+  ///
+  /// \param sig The generic signature.
+  ///
+  /// \returns \c true if a generic signature was appended, \c false
+  /// if it was empty.
+  bool appendGenericSignature(GenericSignature sig);
+
   /// Append a generic signature to the mangling.
   ///
   /// \param sig The generic signature.
   ///
   /// \param contextSig The signature of the known context. This function
   /// will only mangle the difference between \c sig and \c contextSig.
   ///
+  /// \param base The signature of the base entity whose generic signature we're
+  /// mangling. This function will only mangle the inverses on generic
+  /// parameter in \c sig that are not eliminated by conformance requirements in
+  /// \c base.
+  ///
+  ///
   /// \returns \c true if a generic signature was appended, \c false
   /// if it was empty.
   bool appendGenericSignature(GenericSignature sig,
-                              GenericSignature contextSig = nullptr);
+                              GenericSignature contextSig,
+                              BaseEntitySignature &base);
 
   /// Append a requirement to the mangling.
   ///
@@ -527,10 +593,23 @@ class ASTMangler : public Mangler {
   void appendRequirement(const Requirement &reqt, GenericSignature sig,
                          bool lhsBaseIsProtocolSelf = false);
 
+  /// Append an inverse requirement into the mangling.
+  ///
+  /// Instead of mangling the presence of an invertible protocol, we mangle
+  /// their absence, which is what an inverse represents.
+  ///
+  /// \param req The inverse requirement to mangle.
+  void appendInverseRequirement(const InverseRequirement &req,
+                                GenericSignature sig,
+                                bool lhsBaseIsProtocolSelf = false);
+
+  void gatherGenericSignatureParts(GenericSignature sig,
+                                   GenericSignature contextSig,
+                                   BaseEntitySignature &base,
+                                   GenericSignatureParts &parts);
+
   void appendGenericSignatureParts(GenericSignature sig,
-                                   ArrayRef<CanTypeWrapper<GenericTypeParamType>> params,
-                                   unsigned initialParamDepth,
-                                   ArrayRef<Requirement> requirements);
+                                   GenericSignatureParts const& parts);
 
   DependentMemberType *dropProtocolFromAssociatedType(DependentMemberType *dmt,
                                                       GenericSignature sig);
@@ -560,6 +639,7 @@ class ASTMangler : public Mangler {
 
 
   void appendDeclType(const ValueDecl *decl,
+                    BaseEntitySignature &base,
                     FunctionManglingKind functionMangling = NoFunctionMangling);
 
   bool tryAppendStandardSubstitution(const GenericTypeDecl *type);
@@ -575,7 +655,10 @@ class ASTMangler : public Mangler {
   void appendAccessorEntity(StringRef accessorKindCode,
                             const AbstractStorageDecl *decl, bool isStatic);
 
-  void appendEntity(const ValueDecl *decl, StringRef EntityOp, bool isStatic);
+  void appendEntity(const ValueDecl *decl,
+                    BaseEntitySignature &base,
+                    StringRef EntityOp,
+                    bool isStatic);
 
   void appendEntity(const ValueDecl *decl);
 

include/swift/AST/Decl.h

@@ -1856,6 +1856,10 @@ class ExtensionDecl final : public GenericContext, public Decl,
   /// resiliently moved into the original protocol itself.
   bool isEquivalentToExtendedContext() const;
 
+  /// Determine whether this extension context is in the same defining module as
+  /// the original nominal type context.
+  bool isInSameDefiningModule() const;
+
   /// Returns the name of the category specified by the \c \@_objcImplementation
   /// attribute, or \c None if the name is invalid or
   /// \c isObjCImplementation() is false.

include/swift/AST/DeclAttr.def

@@ -491,7 +491,10 @@ DECL_ATTR(_distributedThunkTarget, DistributedThunkTarget,
 DECL_ATTR(_allowFeatureSuppression, AllowFeatureSuppression,
   OnAnyDecl | UserInaccessible | NotSerialized | ABIStableToAdd | APIStableToAdd | ABIStableToRemove | APIStableToRemove,
   157)
-LAST_DECL_ATTR(AllowFeatureSuppression)
+SIMPLE_DECL_ATTR(_preInverseGenerics, PreInverseGenerics,
+  OnAbstractFunction | OnSubscript | OnVar | UserInaccessible | ABIBreakingToAdd | ABIBreakingToRemove | APIStableToAdd | APIStableToRemove,
+  158)
+LAST_DECL_ATTR(PreInverseGenerics)
 
 #undef DECL_ATTR_ALIAS
 #undef CONTEXTUAL_DECL_ATTR_ALIAS

include/swift/AST/Requirement.h

@@ -145,6 +145,11 @@ class Requirement {
     llvm_unreachable("Unhandled RequirementKind in switch");
   }
 
+  friend bool operator!=(const Requirement &lhs,
+                         const Requirement &rhs) {
+    return !(lhs == rhs);
+  }
+
   /// Whether this requirement's types contain ErrorTypes.
   bool hasError() const;
 
@@ -257,6 +262,9 @@ struct InverseRequirement {
 
   InvertibleProtocolKind getKind() const;
 
+  /// Linear order on inverse requirements in a generic signature.
+  int compare(const InverseRequirement &other) const;
+
   /// Appends additional requirements corresponding to defaults for the given
   /// generic parameters.
   static void expandDefaults(ASTContext &ctx,

include/swift/AST/Types.h

@@ -6007,6 +6007,7 @@ class ProtocolCompositionType final : public TypeBase,
   }
 
   InvertibleProtocolSet getInverses() const { return Inverses; }
+  bool hasInverse() const { return !Inverses.empty(); }
 
   void Profile(llvm::FoldingSetNodeID &ID) {
     Profile(ID, getMembers(), getInverses(), hasExplicitAnyObject());

include/swift/Demangling/DemangleNodes.def

@@ -393,5 +393,7 @@ NODE(OutlinedAssignWithTakeNoValueWitness)
 NODE(OutlinedAssignWithCopyNoValueWitness)
 NODE(OutlinedDestroyNoValueWitness)
 
+NODE(DependentGenericInverseConformanceRequirement)
+
 #undef CONTEXT_NODE
 #undef NODE

include/swift/Demangling/TypeDecoder.h

@@ -448,8 +448,14 @@ void decodeRequirement(NodePointer node,
           child->getChild(1), /*forRequirement=*/false);
       if (!constraintType)
         return;
+    } else if (child->getKind() ==
+          Demangle::Node::Kind::DependentGenericInverseConformanceRequirement) {
+      // FIXME(kavon): this is unimplemented! We should build a PCT here with
+      //               the inverse in it.
+      return;
     }
 
+
     switch (child->getKind()) {
     case Demangle::Node::Kind::DependentGenericConformanceRequirement: {
       requirements.push_back(BuiltRequirement(